Scandium is extremely valuable as an additive for high-strength alloys and an electrode material for fuel cells. However, scandium has not yet been used widely due to the small production quantity and high cost thereof.
Meanwhile, a trace amount of scandium is contained in nickel oxide ore such as laterite ore and limonite ore. However, nickel oxide ore has not been used industrially as a raw material for nickel for many years because the grade of nickel in nickel oxide ore is low. Consequently, very few studies also have been conducted for a method of industrially recovering scandium from nickel oxide ore.
Nonetheless, in recent years, the HPAL process has been emerging as a practical method, in which nickel oxide ore is introduced into a pressure vessel along with sulfuric acid, and heated at a high temperature of 240° C. to 260° C. to allow solid-liquid separation into a nickel-containing leachate and a leach residue. In the HPAL process, a neutralizing agent is added to the leachate obtained to separate impurities, and then a sulfurizing agent is added to the resulting leachate from which impurities are separated out, allowing recovery of nickel as nickel sulfide. Subsequently, this nickel sulfide may be subjected to a known nickel refinement process to obtain electrolytic nickel and nickel salt compounds.
In the HPAL processes as described above, scandium contained in nickel oxide ore will be contained in a leachate along with nickel (see Patent Document 1). Subsequently, when a neutralizing agent is added to a leachate obtained from the HPAL process to separate impurities, and a sulfurizing agent is then added, nickel is recovered as nickel sulfide while scandium remains in the acidic solution after addition of the sulfurizing agent. In this way, nickel can effectively be separated from scandium by using the HPAL process.
A method of separating scandium by using a chelating resin has also been documented (see Patent Document 2). Specifically, according to the method disclosed in Patent Document 2, nickel-containing oxide ore is first treated at high temperature and high pressure under an oxidizing atmosphere to selectively extract nickel and scandium into an acidic aqueous solution. Subsequently, the pH of the resulting acidic solution is adjusted to the range of 2 to 4, and nickel is then selectively precipitated and recovered as a sulfide by means of a sulfurizing agent. Next, the resulting solution from which nickel has been recovered is allowed to make contact with a chelating resin, and the chelating resin is washed with a dilute acid. Then, the chelating resin which has been washed is allowed to make contact with a strong acid to elute scandium from the chelating resin.
Further, as a method of recovering scandium from the aforementioned acidic solution, the method of recovering scandium by means of solvent extraction has also been proposed (see Patent Documents 3 and 4). Specifically, according to the method disclosed in Patent Document 3, an organic solvent is first added to an aqueous-phase scandium-containing solution to extract a scandium component into the organic solvent, the organic solvent comprising 2-ethylhexyl sulfonic acid-mono-2-ethylhexyl diluted with kerosene, and the aqueous-phase scandium-containing solution containing one or more of at least iron, aluminum, calcium, yttrium, manganese, chromium, and magnesium in addition to scandium. Then, in order to separate yttrium, iron, manganese, chromium, magnesium, aluminum, and calcium extracted into the organic solvent along with scandium, an aqueous solution of hydrochloric acid is added, and scrubbing is performed to remove these elements. Then, an aqueous solution of NaOH is added to the organic solvent to transform scandium remaining in the organic solvent into a slurry containing Sc(OH)3, and the slurry is filtered to obtain Sc(OH)3, which is then dissolved in hydrochloric acid to obtain an aqueous solution of scandium chloride. Subsequently, oxalic acid is added to the resulting aqueous solution of scandium chloride to obtain a precipitate of scandium oxalate. This precipitate is filtered to remove iron, manganese, chromium, magnesium, aluminum, and calcium into a filtrate, and then calcination is performed to obtain highly pure scandium oxide.
Moreover, Patent Document 4 describes a method of selectively separating and recovering scandium from a scandium-containing supply liquid, the method comprising: bringing the scandium-containing supply liquid into contact with an extracting agent at a certain ratio in a batch process.
The grade of scandium recovered according to these methods is known to be about 95% to 98% pure in terms of scandium oxide. The above grade may be good enough for those uses such as an additive in alloys. However, a much higher purity, for example, the purity of about 99.9%, is required as a grade used for electrolytes of fuel cells which have recently much in demand. Otherwise, their full capability may not be obtained.
However, nickel oxide ore described above contains various impurity elements such as manganese and magnesium in addition to iron and aluminum, and may even contain a trace amount of actinoid elements, such as uranium and thorium, and others although impurities may somewhat vary in identities and their contents, depending on mining regions.
The acceptable upper limit of the grade of an impurity element may be pre-determined for scandium to be used in electrolytes of fuel cells and the like as described above. Therefore, each element may need to be individually separated out to the level of the acceptable limit or below.
However, the uses of the chelating resin and the organic solvent disclosed in Patent Documents 2 and 3 may not effectively separate some of the aforementioned impurity elements, in particular actinoids, from scandium because some of the aforementioned impurity elements may show similar behaviors as scandium. Further, impurities to be contained in the leachate of nickel oxide ore, such as iron and aluminum, have much higher concentrations than scandium. Due to these large amounts of impurities and other effects, a method suitable for industrially recovering high purity scandium from nickel oxide ore has not been found.
Further, scandium is preferably recovered in a solid form in order to facilitate the use of scandium as a product. To this end, a process for obtaining precipitates of scandium hydroxide and scandium oxalate needs to be performed, the process comprising adding a neutralizing agent such as an alkaline substance; and oxalic acid to a solution containing scandium obtained by the techniques disclosed in the aforementioned Patent Documents.
However, for example, some or most of impurities such as aluminum and iron contained in the solution are also precipitated at the same time when an alkaline substance is added to recover scandium in a form of hydroxide. This makes selective separation of scandium difficult. Further, a hydroxide of scandium may disadvantageously take a gel form with inferior filterability, resulting in decreased handling properties such as long filtering time.
In contract, for example, according to the method of recovering scandium as an oxalate salt as disclosed in Patent Document 5, the method comprising: adding oxalic acid ((COOH)2) to a scandium-containing solution, the resulting scandium oxalate has good filterability, showing an advantage of better handling properties.
However, the scandium-containing solution obtained from nickel oxide ore as decreased above contains a large amount of impurities such as aluminum and iron from ore components, and thus a large amount of precipitates of aluminum oxalate and ferrous oxalate (II) are also generated, again resulting in difficult recover of scandium. In addition, this process suffers from an increased cost of oxalic acid to be used.
Accordingly, an oxidizing agent is added to the scandium-containing solution to oxidize iron ions contained in the solution into a trivalent form. This promotes formation of highly soluble ferric oxalate (III), thereby preventing precipitation of iron. This process, however, suffers from an increased cost of the oxidizing agent.
As described above, effective recovery of high purity scandium from nickel oxide ore is difficult because a wide variety of impurities need to be separated out such as iron and aluminum, which are contained in large amounts, and actinoid elements.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. H03-173725
Patent Document 2: Japanese Unexamined Patent Application, Publication No. H09-194211
Patent Document 3: Japanese Unexamined Patent Application, Publication No. H09-291320
Patent Document 4: PCT International Publication No. WO2014/110216
Patent Document 5: Japanese Unexamined Patent Application, Publication No. H09-248463